The present invention relates to electrostatographic toners and, more particularly, to a process for forming yellow toners using the dry aggregated state of an isoindoline yellow pigment.
Pigment concentrates, or pigment predispersions, are used to prepare toners employed in full color electrostatographic processes. Melt blending of dry pigment materials with other toner ingredients such as binder resins, charge agents, wax additives and the like, generally using a roll mill or an extruder, typically results in a nonuniform dispersion of the pigment in the melt. Subsequent cooling and pulverization of the melt produces a toner having poor color quality and low tinctorial strength. Uniform dispersion of a colorant or pigment in a color toner is necessary for realizing the highest possible color gamut, minimizing the amount of toner laid down on a page, achieving reproducibility of color and electrostatic and physical properties, and optimizing the cost of toner preparation.
In order to overcome the aforementioned dispersion difficulties, it is common practice in the prior art to prepare a pigment concentrate or predispersion by a technique known as xe2x80x9cflushing,xe2x80x9d which entails using a vehicle that is compatible with the final toner binder resin to remove water from a damp pigment press cake. The flushed pigment concentrate is then added to the melt blending step of the toner manufacturing process, resulting in a relatively good dispersion of the pigment.
The flushing process and its advantages as applicable to printing inks are described in The Printing Ink Manual, Third Edition, D. E. Bisset, C. Goodacre, H. A. Idle, R. H Leach and C. H. Williams, editors, Northwood Publications Ltd, London, 1979, page 313:
xe2x80x9cFlushing is defined as the operation in which the water phase of an aqueous pigment press cake is removed and replaced by an oil or resin phase, such as an ink vehicle, in one operation. This avoids the normal operations of drying, crushing and powdering the pigment, and redispersing in a selected ink vehicle. The flushing process starts with the pigment in press cake form; therefore the pigment particles have not been subjected to a heat-assisted drying process which can lead to the formation of hard aggregates. Thus, the degree of dispersion of flushing is usually very good, but the process is suited only to production of large batches for reasons of cost. Advantages claimed for the flushing process are that some pigments, such as diarylide yellows, are heat sensitive and flushing overcomes the difficulties that occur when drying these pigments normally. Claimed advantages include greater transparency, better gloss and higher tinctorial strength of flushed colors.xe2x80x9d
U.S. Pat. No. 4,759,801, the disclosure of which is incorporated herein by reference, describes a kneading or flushing process starting with a wet pigment press cake that is asserted to have advantages of energy savings, lower temperatures enabling the use of heat sensitive pigments, higher concentration preparations, and high color yield and purity compared to previously known methods in which finely-ground dry pigment is mixed with thermoplastic material, i.e. synthetic waxes or synthetic resins, at extrusion temperatures of up to 300xc2x0 C.
The diarylide family of yellow pigments as a class has found wide application in process color printing inks, members of which have also found use in electrostatographic toner based applications for process color copying and printing. However, this class of pigments is synthesized from a toxic intermediate known as dichlorobenzidine, which itself can be generated by thermally decomposing such pigments. Modern industrial practice is to avoid using such materials if at all possible.
Thus, there has been a need to find high strength alternatives to diarylide yellow colorants for use in electrostatographic full process color printing processes that employ a toner set comprising a cyan, a magenta, a yellow and, optionally, a black toner. One such alternative is the isoindoline yellow pigment known by its Color Index designation as Pigment Yellow 185, or P.Y. 185. As stated in Industrial Organic Pigments, W. Herbst and K. Hunger, VCH Verlagsgellschaft Weinheim, Germany, 1993, at page 412: xe2x80x9cP.Y.185, a comparatively recent product, is an isoindoline pigment and provides clean greenish shades of yellow. Its main field of application is in packaging printing inks. P.Y. 185 confers good tinctorial strength and high gloss on prints made from NC-based inks. Its tinctorial strength in print exceeds that of P.Y.17, a pigment of the diarylide yellow pigment series, which provides the same hue.xe2x80x9d
Flushed isoindoline yellow pigment concentrates, including P.Y.185, have been used, along with other flushed pigment concentrates to prepare cyan, magenta, yellow and black process color toner sets, as described in U.S. Pat. Nos. 5,554,471, 5,607,804 and 5,620,820, the disclosures of which are incorporated herein by reference.
The present invention is directed to a process for forming a particulate yellow toner for the development of latent electrostatographic latent images. The process comprises: combining dry Pigment Yellow 185 (P.Y.185) and a first thermoplastic polymer under conditions effective to form a predispersion of P.Y.185 in the polymer, and combining the predispersion with a second thermoplastic polymer under conditions effective to form a yellow toner composition, from which is formed yellow toner particles.